The invention relates to a method for operating a powertrain in a motor vehicle, which powertrain has an internal-combustion engine and components for coupling the internal-combustion engine to the drive wheels of the motor vehicle in a controllable manner.
In the case of hybrid vehicles available today, the internal-combustion engine is switched off at least partially under specified operating conditions when the vehicle is driving, and the vehicle is driven only by an electric motor. This so-called “sail” mode, known in the case of hybrid vehicles while the internal-combustion engine is switched off, is to be used in the future also in the case of conventional vehicles without an electric hybrid drive.
Thus, from International Patent Document WO 02/094601, a method for operating a vehicle in a sail mode is known, in which, in coasting operating phases, in which neither the gas pedal nor the brake pedal is actuated, the internal-combustion engine is decoupled from the drive wheels. In this case, the rotational engine speed of the internal-combustion engine decreases to a rotational idling speed. As a result, the drag torques are reduced, whereby fuel savings are to be expected.
As a result of the separation of the internal-combustion engine from the powertrain or from the drive wheels and the falling of the rotational engine speed back to the defined rotational idling speed, shortfalls in the electric onboard power supply system may occur, because the electric energy generation, in the case of conventional motor vehicles having an internal-combustion engine, takes place predominantly by belt-driven generators. These generators supply more current as the rotational speed of the internal-combustion engine rises. Low rotational speeds—as here in the idle sail mode—may result in a so-called shortfall in the onboard power supply system, because a shortfall will already exist when the electric power generated prior to the sail mode is greater than the electric power generated in the idle sail mode. Depending on the amount of the shortfall as well as the duration and frequency, this may result in an increased wear of the battery as well as in a deterioration of the charge balance.
From the type-forming German Patent Document DE 10 2009 057 551 A1, a method is known for operating a powertrain in a motor vehicle, in which, when predefined operating conditions exist when the vehicle is driving, the internal-combustion engine is decoupled from the drive wheels; thus, a change takes place to a sail mode. The above-mentioned document further discloses that a sail mode can be set only when the charge of an electric power accumulator is not too low.
It is an object of the invention to provide a method for operating a vehicle in a sail mode while taking into account the providing of sufficient electric power.
This and other objects are achieved by a method according to the invention for operating a powertrain in a motor vehicle, which powertrain comprises an internal-combustion engine and components for the controllable coupling of the internal-combustion engine with the drive wheels of the motor vehicle, wherein when predefined operating conditions are present, in the case of the traveling motor vehicle, the internal-combustion engine is decoupled from the drive wheels in order to allow the vehicle to coast. The predefined operating conditions may be a coasting of the motor vehicle during which neither the gas pedal nor the brake pedal is actuated. As a further limitation, a decoupling may only be initiated when the speed of the vehicle is within a predefined speed window. Simultaneously with the decoupling of the drive wheels or after the decoupling of the drive wheels, the internal-combustion engine is controlled such that the rotational engine speed decreases to a predefined desired rotational idling speed.
This invention is based on the recognition that sufficient electric power can be provided only by a corresponding rotational engine speed of the internal-combustion engine and therefore a rotational speed of the internal-combustion engine which meets the requirements is to be set in the decoupled operation.
The invention is characterized in that the desired rotational idling speed is predefined or influenced in a variable manner depending on the situation; i.e. after the decoupling, the rotational engine speed is not set to a rotational idling speed that is always identical irrespective of different influence factors, but different demands and operating conditions can be taken into account as a result of the variability.
Advantageously, the desired rotational idling speed is determined as a function of the actual operating state of a generator coupled with the internal-combustion engine for generating current while the powertrain is still coupled, in order to thereby be able to prevent or counteract a shortfall occurring during the decoupling of the drive wheels. Since the generator current supply depends on the generator temperature, the rotational generator speed and the generator type, the desired rotational idling speed is advantageously determined as a function of the actually generated current, of the generator temperature while the powertrain is still coupled and/or the generator type. In particular, for calculating the desired rotational idling speed for the sail mode, the respective characteristic generator curve can be used; i.e. the desired rotational idling speed is determined from the type-specific and temperature-specific characteristic generator curve, in which case the actual current is used as the input quantity and the rotational engine speed required for this purpose is determined.
Depending on the amount of the determined desired idling speed, a sail mode is now either set or not set. Thus, in an advantageous further development of the invention, when the predetermined operating conditions are present, the desired rotational idling speed is determined and the internal-combustion engine is decoupled from the drive wheels and the rotational engine speed is set to the determined desired rotational idling speed when the determined desired rotational idling speed is within a predefined (configurable) rotational idling speed window or the determined desired rotational idling speed is lower than a predefined rotational idling speed threshold. The setting of the desired rotational idling speed or of a permissible rotational speed determined from the desired rotational idling speed because of clustering takes place by means of a rotational (idling) speed control element or an automatic rotational (idling) speed control element.
When the determined desired rotational idling speed is not within the predefined rotational idling speed window or the determined desired rotational idling speed is not lower than a predefined rotational idling speed threshold, the setting of the sail mode will not be useful; i.e. the decoupling of the internal-combustion engine from the drive wheels is prevented by placing a so-called sail preventor in the control unit. However, when a falling-below the generator current is not detected before the sail mode is taking place, this will not be taken into account and the sail mode will be continued.
As an additional measure, with or after the initiation of the sail mode, thus, with or after the decoupling of the internal-combustion engine from the drive wheels, a deactivation of different electric power consuming devices can also be carried out and/or predefined electric power consuming devices can be triggered or automatically controlled such that the power consumption will be reduced. A conceivable shortfall in the onboard power supply system can therefore also be positively influenced.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.